The present invention relates to a color cathode ray tube, and more particularly to a so-called flat panel type color cathode ray tube which is provided with a panel whose outer surface has the radius of curvature extremely larger than that of an inner surface thereof.
Recently, color cathode ray tubes called the flat face type or the flat panel type have been widely adopted as picture tubes for television receivers or monitor display tubes of personal computers or the like.
Generally, a glass-made envelope of a color cathode ray tube is composed of a panel which forms a display part (screen), a narrow-diameter neck and a funnel which connects the panel with the neck. A phosphor screen (a phosphor film) coated with a phosphors in three colors is formed on the inner, surface of the panel and a shadow mask which works as a color selection electrode is disposed close to this phosphor screen.
Further, an electron gun which emits three electron beams is accommodated in the inside of the neck and three electron beams emitted from the electron gun are made to pass through beam apertures formed on the shadow mask and are impinged on respective phosphors to reproduce color images.
Recently, this type of color cathode ray tube has an outer surface of the panel thereof flattened so as to enhance the visibility thereof. Particularly, this flattening of the panel is widely adopted with respect to color cathode ray tubes having large screens. The color cathode ray tubes having such flattened panels are called the flat panel type color cathode ray tubes.
As documents which disclose conventional techniques on this kind of flat panel type color cathode ray tube, Japanese Laid-open Patent Publication 9-245685 and Japanese Laid-open Patent Publication 11-238475 can be named.
FIG. 5 is a schematic cross-sectional view showing a structural example of an essential-part of a flat panel type color cathode ray tube. In the drawing, a panel 1 is joined to a large diameter periphery which constitutes one end of a funnel 3 and the other end of the funnel 3 which gradually narrows its diameter in a funnel shape is connected to a neck not shown in the drawing.
An outer surface PO of the panel 1 which forms a phosphor screen 15 having an approximately rectangular shape on an inner surface thereof has a curved surface which is substantially a flat surface, wherein the radius of curvature of a curved surface of the inner surface PI is set to be smaller than the radius of curvature of the outer surface PO for maintaining the mechanical strength of a glass-made envelope.
A shadow mask 6 is disposed in the vicinity of this phosphor screen 15. A large number of electron beam passing apertures 6a are formed in the shadow mask 6. The shadow mask 6 is welded to a mask frame 7 and is held to the inner surface of a side wall of the panel 1 by way of a suspension mechanism not shown in the drawing.
In view of the manufacturing cost and the easiness of manufacturing, with respect to the above-mentioned flat panel type color cathode ray tube, the outer surface (also called xe2x80x9cfacexe2x80x9d) of the panel is set to have a large radius of curvature, that is, the outer surface is set to an approximately flat surface, while the inner surface on which a phosphor layer is formed is set to have a relatively small radius of curvature to a degree that the feeling of a flatness of a displayed image is not spoiled when the display screen is seen from the outer surface.
In manufacturing the flat panel type color cathode ray tube, it is easy to approximate the shape of the outer surface of the panel to the flat surface. However, to approximate the inner surface of the panel to the flat surface, the thickness of the whole panel must be considerably increased to increase the mechanical strength of the glass-made envelope. Therefore, this is not practical in view of the increase of the weight of the cathode ray tube, the increase of cost and the like.
Further, on the other hand, with respect to the shadow mask which is not a color selection electrode of a so-called tension type, it is necessary to form a mask surface thereof with a certain degree of curvature while eliminating a completely flat surface to make the shadow mask stand by itself. Since the manufacturing of a shadow mask having large radius of curvature by a press molding is technically limited, it is necessary to give a given curvature to the shadow mask and simultaneously to give a given curvature to the inner surface of the panel.
As shown in FIG. 5, the curvature of the inner surface PI of the panel 1 is larger than the curvature of the outer surface PO (the radius of curvature of PI being smaller than the radius of curvature of PO) and the shadow mask 6 approximately traces the shape of the curved surface (warp) of the inner surface PI of this panel so that the flatness is deteriorated at the end peripheries in the long axis side (X axis) on the screen 15.
In the flat panel type which largely warps the inner surface, there has been a problem that the larger the panel size of the color cathode ray tube (equal to or more than xe2x80x98nominal 17 inchesxe2x80x99 (effective screen diagonal diameter being 41 cm)) becomes, the feeling of flatness in the short sides of the screen (end portions in the X axis direction) is worsened due to the aspect ratio of the screen.
Further, in the above-mentioned flat panel type color cathode ray tube, the shape of the curved surface of the shadow mask 6 per se approximately follows the shape of the inner surface PI of the panel 1. Accordingly, although it may be certain that the radius of the curvature of the inner surface PI of the panel 1 is actually set smaller than the radius of curvature of the outer surface PO, of the panel 1, compared to a conventional spherical panel type color cathode ray tube, the flatness of the inner surface PI of the panel 1 and the shadow mask 6 is made extremely greater.
Accordingly, when a display monitor is completed by assembling the color cathode ray tube in the display monitor, there has been a problem that the shadow mask is liable to be easily deformed against an external impact. That is, the flatter the shape of the curved surface of the shadow mask 6 is, the shadow mask is liable to be deformed more and this has been one of factors which impede the enhancement of the quality.
The typical object of the present invention is to provide a color cathode ray tube provided with a panel which enhances the visual characteristics and a shadow mask which exhibits a high resistance against a mechanical impact.
According to the typical aspect of the present invention, focusing on a finding that the visual characteristics of an approximately rectangular effective screen (phosphor screen) formed on a panel of a color cathode ray tube dominantly depends on the radius of curvature in the short side direction, that is, the short axis (Y axis) direction of the panel in view of the aspect ratio, the shape of the curved surface of the panel in the phosphor screen region and the shape of the curved surface of the shadow mask in the apertures region are constituted as follows.
(1) The curvature along the diagonal direction of the outer surface of the panel approximates to flat, the equivalent radius Rox of curvature along the long axis (X axis) of the outer surface of the panel is set smaller than the equivalent radius Roy of curvature along the short axis (Y axis) of the outer surface of the panel and the equivalent radius Rix of curvature along the long axis (X axis) of the inner surface of the panel is set larger than the equivalent radius Riy of curvature along the short axis (Y axis) of the inner surface of the panel.
(2) In accordance with a result of an analysis, a suitable value of a ratio between the equivalent radius of curvature along the Y axis of the inner surface of the panel and the equivalent radius of curvature along the X axis of the inner surface of the panel is set to fall in the range of 0.7 less than (Riy/Rix) less than 1.0.
(3) The equivalent radius Rol of curvature along the long side portions of the outer surface of the panel is set smaller than the equivalent radius Ros of curvature along the short side portions of the outer surface of the panel and the equivalent radius Ril of curvature along the long side portions of the inner surface of the panel is set larger than the equivalent radius Ris of curvature along the short side portions of the inner surface of the panel.
(4) In accordance with a result of an analysis, a suitable value of a ratio between the equivalent radius of curvature along the short side portions of the inner surface of the panel and the equivalent radius of curvature along the long side portions of the inner surface of the panel is set to fall in the range of 0.7 less than (Ris/Ril) less than 1.0.
(5) The arrangement pitch in the horizontal direction of the electron beam passing apertures formed in the shadow mask is set to a variable pitch which becomes larger from the central portion to the peripheral portions (end portions) along the X axis.
(6) The equivalent radius Ros of curvature along the short side portions of the outer surface of the panel is set equal to or more than 10000 mm.
In the above-mentioned respective constitutions, particularly, by setting the equivalent radius Rox of curvature along the X axis of the outer surface of the panel smaller than the equivalent radius Roy of curvature along the Y axis of the outer surface of the panel or by setting the equivalent radius Rol of curvature along the long side portions of the outer surface of the panel smaller than the equivalent radius Ros of curvature along the short side portions of the outer surface of the panel, the visual characteristics, that is, the feeling of flatness of the screen which is one of the tasks of the present invention is greatly enhanced.
Further, by setting the equivalent radius Rix of curvature along the X axis of the inner surface of the panel larger than the equivalent radius Riy of curvature along the Y axis of the inner surface of the panel or by setting the equivalent radius Ril of curvature along the long side portions of the inner surface of the panel larger than the equivalent radius Ris of curvature along the short side portions of the inner surface of the panel, the impact resistance characteristics, that is, the mechanical strength which is one of the tasks of the present invention is greatly enhanced.
By setting the suitable value of the ratio between the equivalent radius of curvature Riy along the Y axis of the inner surface of the panel and the equivalent radius of curvature Rix along the X axis of the inner surface of the panel such that the value falls in the range of 0.7 less than (Riy/Rix) less than 1.0, the mechanical strength of the shadow mask is further reinforced and the impact resistance in the drop test of the completed color cathode ray tube is greatly enhanced.
Due to such constitutions, the flat panel type color cathode ray tube which has the improved visibility and can suppress the deformation of the shadow mask due to the external impact can be obtained.
Although operations and effects of the above-mentioned typical. constitutions will be explained in detail in the paragraphs of embodiment, it is needless to say that the present invention is not limited to those described in the embodiment and various modifications can be considered without departing from the technical ideas of the present invention.